Accelerometer



y 4, 1967 5. M. DAVIDSON ETAL 3,329,027

ACCELEROMETER Filed Sept. 20, 1956 I 2 Sheets-Sheet 1 I5 wig. 1.

SHIF'TE'R NULL NETM I? I I F'jg. 3.

INVENTORS. GA'FEETH M. DAVIDSON FRANKLIN MEYER Ww/fi ZATTOENCY y 4, 1967a. M. DAVIDSON ETAL 3,329,027

ACCELEROMETER Filed Sept. 20, 1956 2 Sheets-Sheet 2 all EI/CLK DEMO-//V\/E/V7'OP8. GA'EETH M. DAVIDSON FRANKLIN MEYER ATTOPA/f).

United States Patent 3,329,027 ACCELEROMETER Gareth M. Davidson, Bronx,and Franklin Meyer, Franklin Square, N.Y., assignors to American BoschAnna Corporation, a corporation of New York Filed Sept. 20, 1956, Ser.No. 611,042 6 Claims. (Cl. 73-517) The present invention relates toaccelerometers, and has particular reference to vibrating stringaccelerometers.

A vibrating string accelerometer has been previously described in aco-pending US. patent application Ser. No. 586,615, filed May 22, 1956,in the name of Charles D. Bock; Paul S. Jorgensen and Joseph Statsingerfor Accelerometer, and assigned to the assignee of this invention. Withthe arrangement there disclosed, a relatively complex computing means isemployed to compensate for the non-linear relationship between thefrequency change of a vibrating string and the acceleration of thecraft. The present invention seeks to avoid this complexity, and iscapable of providing a beat frequency output which is directlyproportional to the applied acceleration.

The basic mechanical structure of the present accelerometer includes apair of wires isolated by a central mass with one wire being solidlyfixed at one end of a frame and the other wire being connected to atransducer fixed to the frame for varying the overall tension of thewires. During operation, both wires are excited into transversevibration at their natural frequencies by electronic means, whichfrequencies are nominally equal in the absence of an externaldisturbance. If a system acceleration is applied in the direction of thewires, then a difference in tension in the wires results with acorresponding difference in the natural vibration frequencies, which arecompared to produce a beat frequency output. Without an externalcompensation system, the magnitude of the beat frequency is a non-linearfunction of the applied acceleration.

The compensation provided by the present application is such as to keepthe sum frequency constant by continuous monitoring of the initialtension so that the beat frequency is directly proportional in theacceleration, as will be shown.

In accordance with this invention, the two frequencies are fed into anon-linear network such as a demodulator and the sum frequency isfiltered out. The sum frequency is then compared with an accuratestandard reference frequency in a frequency discriminator. An output isproduced in the discriminator when the frequencies of the two inputsignals are not exactly equal. This is fed into a high gain amplifierwhich feeds the tension-control transducer to vary the wire tension in amanner which tends to maintain the sum frequency equal to the referencefrequency. Various types of tension control elements are available. Forvery fast response the use. of a magnetostrictive transducer ispreferred. Changes in the characteristics of the control element havenegligible effect on the overall accuracy of the system, since the basicreference is an accurately controlled standard frequency.

For a more complete understanding of the invention, reference may be hadto the accompanying diagrams, in which,

FIG. 1 is a schematic diagram of the accelerometer system;

FIG. 2 illustrates one type of tension control transducer;

FIG. 3 illustrates another tension control transducer;

FIG. 4 is an alternative embodiment of the tension control;

FIG. 5 illustrates another possible embodiment of the invention; and

FIG. 6 is a sectional view through line 66 of FIG. 5.

FIG. 1 shows schematically the general arrangement of "ice the vibratingstring accelerometer of this invention. The transducer 10 includes apair of electrically conducting wires 11, 12 which are attached to amass 13 and stretched between the supports 14 of a suitable frame 15.The mass 13 is additionally supported in the frame 15 by a plurality ofsymmetrically placed suspension members 16. Permanent magnets 17 and 18are placed so as to provide transverse magnetic fields across the wires11 and 12 respectively in mutually perpendicular directions. Each of thewires 11, 12 is placed in the circuit of its corresponding amplifieroscillator 19, 20 which is effective in keeping the wire 11 or 12vibrating at its natural frequency as determined by the physicalconstants of the wire and the tension under which it is stressed. Suchvibrating wire oscillators are not new, per se, and have been suggestedfor use in variable frequency standards for example.

In an accelerometer employing the vibrating string resonator, theacceleration force acting on a mass varies the natural frequency of thewire in accordance with a non-linear function of the acceleration. Oneremedy for this situation is described in the co-pending US. patentapplication Ser. No. 586,615 previously referred to. The presentinvention herein described is an improved arrangement whereby thecomplex remedial means of that application is made unnecessary. Inaccordance with the concepts of this invention, the tension in the wire11, 12 is continuously monitored to keep the sum of the vibrationfrequencies of the wires 11, 12 at a constant value.

To this end an electromechanical transducer 21 is interposed between atleast one wire 11 or 12 and the support 14. The transducer 21 may takemany forms, several of which will be described, and any of which isadapted to adjust the tension in the wires 11, 12 in accordance with anelectrical signal applied to the transducer 21. By this device, the sumfrequency can be kept at a constant value under acceleration or underenvironmental changes, and if the sum frequency is kept constant, thedifference frequency is a true indication of the acceleration as will beevident from the following discussion.

If it is assumed that the two wires 11 and 12 are iden tical in freelength, l, and mass per unit length, m, the expressions for thefrequency of vibration of each wire is where T and T are the tensions ineach wire 11 and 12. The values of T and T are respectively where To isthe initial tension with no external disturbance, a is the magnitude ofthe acceleration applied in the direction of the wires and M is the massof the weight 13. Therefore, it can be written that and f The product ofthe sum and differences of (l) and (2) yields:

It will be seen by Equation 3 that a value proportional to theacceleration and independent of the initial tension is produced bymultiplication'of the sum and difference frequencies. If now the sumfrequency is kept constant, by

1 controlling the initial tension, the difference frequency f f isdirectly proportional to the acceleration a. In the present instancethis is accomplished by a nulltype or closed loop control in the mannerto be described.

In the following discussion, the terms f output, f component or similarterms should be interpreted as signifying alternating voltages ofconstant amplitude and frequency equal to fa. The f output of amplifieroscillator 19 and the i output of amplifier oscillator 20 are applied toa frequency summing device 22 which produces an output voltage having afrequency 1, equal to the sum of the frequencies of its input signals, f+f An illustrative summing device for this purpose is described in theco-pending patent application previously referred to, although any otherdevice performing the required summation may be used, if desired.

The device described in the co-pending patent application uses areference voltage having a frequency f which is much higher than eitherf or fig, and a pair of demodulators, one having inputs of fr and f andthe other having inputs of f, and i The f,+f output component of thefirst demodulator can be separated from the others by filter means, andthe f -f component of the second demodulator output can be similarlyobtained. The f -l-f voltage and the fl-f voltage are applied to a thirddemodulator and filter for obtaining a difference frequency component,whence the output component has a frequency (fr+f11) (fr f12) f11+f12Which is the desired frequency sum.

The (f +f output of the summing device 22 is applied to a frequencydifference device 23 which is also supplied with a reference voltagehaving a frequency f from the frequency standard 24. The frequency f ischosen to be a given amount f greater than f -l-f at zero accelerations.

Under zero acceleration, then, the output of the frequency differencedevice 23 is equal to f and the sum frequency f +f is known as theinitial su-m frequency. The nominal is output of the difference device23 is applied to a discriminator or null network 25 which has acharacteristic such that the magnitude of its output voltage is zero forsignals of fc frequency, and over a limited range the magnitude of theoutput voltage is proportional to the error f between the value of f (f+f and f A typical null network for this purpose is the twin-T networksuch as that shown in the rectangle marked 25.

The output of the network 25 is amplified in the amplifier 26 andrectified in the demodulator 27 to provide a DO. signal proportional inmagnitude to the null network output. In order to provide phasesensitivity to the demodulator 27, a reference voltage of the correctfrequency and substantially constant magnitude is taken from the outputof the frequency difference device 23. A phase shifting network 28 isinterposed between the output of device 23 and the reference input tothe demodulator 27 and is necessary to compensate for the phase shiftintroduced by the null network 25.

The DC. output of the demodulator, now proportional in magnitude anddirection to the difference between the actual sum frequency of thewires 11 and 12 and the initial sum frequency, is applied to theelectromechanical transducer 31 which is adapted to vary the tension ofthe wires 11 and 12 in accordance with the error between the actual andinitial sum frequencies, and to thereby reduce this error to zero.

As mentioned earlier, it is contemplated that the transducer 21 may takemany forms. FIG. 2, for example, shows one embodiment where the wire 12is attached to a plug 30 which is adapted to be positioned with respectto the frame 15 by a servo motor 31. By means of gearing 32, motor 31drives the screw shaft 33 which is threaded in the plug 30, the plugbeing non-rotatable in the frame 15. Motor 31 is energized by the outputof the demodulator 29 whence motor positions the plug 30 to change thetension in the wires 11, 12 until the sum frequency of the wires 11, 12is equal to the desired initial value so that the output of demodulator27 is zero and motor 31 is deenergized.

It will be seen that this means will also be effective in eliminatingsuch wire tension changes as may be caused by temperature variation,mechanical creep and other environmental conditions, and which wouldproduce errors if not compensated for by other means.

Another embodiment of the transducer 21 is shown in FIG. 3 where amagnetostrictive member 40 located in the magnetic field of a solenoid41 is interposed between the wire 12 and the frame 15. The winding ofthe solenoid 41 is connected to be energized by the output of thedemodulator 27. Magnetostrictive elements have the property of changingdimensions under the application of magnetic fields, and this propertycan be used to adjust the wire tension.

Under acceleration, the sum frequency of wires 11, 12 changes in adirection such that an increase in tension is required to return the sumfrequency to the initial value. Since the magnetostrictive element 40always operates to increase its dimensions and thereby decrease the wiretension, no matter what the direction of the applied field, certainarrangements must be employed to get the desired reaction. For thisreason the magnetostrictive element 40 is operated with a bias magneticfield in order to get motion in either direction.

The required bias field is preferably applied by making the initialnon-operating tension in the wires 11, 12 much higher than the desiredinitial operating tension. The resulting deviation of the sum frequencyfrom that prescribed by the reference frequency supply causes an outputin the demodulator 26 such that the magnetostrictive element 40 expandsto reduce the wire tension. A point of equilibrium will be reached whenthe demodulator output is sufficient to keep the sum frequency at therequired value. Other means of biasing may be used if convenient, andthe invention should not be limited by the embodiment described.

It is evident to those skilled in the art that other configurations oftransducers may be employed such as those employing members which bendor twist for example under application of magnetic or electric fieldswith appropriate mechanical structure, or which produce proportionalmechanical movement in response to electrical signals in any mannerwhatsoever.

In a further alternative, there are two tension control transducers 21a,21b one for each wire 11 and 12, as shown in FIG. 4. This arrangementhas the advantage that the transducers 21a, 21b effectively stretch thewires from both ends, so that the weight 13 and the suspension members16 remain in constant relation to frame 15. In the electrical circuit,the transducers 21a and 2112 would both be energized by the same signal,i.e., the output of the demodulator 27. The transducers 21a and 2112 canbe connected either in parallel or in series whichever is mostdesirable.

FIG. 5 shows a sectional view of another possible embodiment in whichthe frame 15 includes magnetostrictive legs 42 and 43 whereby the entireends 44, 45 of the frame 15 are moved relatively to one another toeffect the tension variation in wires 11, 12. In this embodiment, themagnetostrictive portions 42, 43 of the frame 15 are located within therespective twin solenoids 46, 47, the windings of which may be connectedin series or parallel, whichever is found most desirable, across theoutput of demodulator 27. The vibrating string assembly of FIG. 5 ispreferably mounted on the object whose acceleration is of interest bythe center of the magnetostrictive elements 42, 43, since both ends 44,45 must be free and the center weight 13 should be maintained stationarywith respect to the object. The end pieces 44, 45 are of magneticmaterial to provide the return path for the magnetic flux.

To this end a mounting ring 48, which may have a T shaped cross section,is employed to support the frame 15 on the object and to support thesolenoids 46, 47. It should be pointed out that the representation ofFIG. 5 is merely schematic and illustrative, and that the physicalembodiment of the frame most likely contains three or more longitudinalpieces with spider shaped end pieces having an appropriate number ofradial arms. For example, a section taken through line 66 of FIG. 5 mayappear as shown in FIG. 6. However, the principle here described may beapplied to any configuration by those skilled in the art Withoutdeparting from the invention.

When the sum frequency is kept constant as herein described, thedifference frequency as obtained in a demodulator filter arrangement 29will be directly proportional to the acceleration of the support. Acount of the beat frequency pulses will therefore be proportional to theintegral of the acceleration, or to the velocity of the craft.

We claim:

1. In a device of the character described, a frame, a weight suspendedin said frame by a plurality of suspension members, a pair of tensionmembers connected between said weight and said frame and means forvarying the tension of said tension members, according to the differencebetween a fixed frequency and the sum of the natural frequencies of saidtension members, means for obtaining the sum of the natural frequenciesof said tension members and means for energizing said tension varyingmeans to keep the sum of said natural frequencies at a constant value.

2. In a device of the character described, a frame, a weight suspendedin said frame by a plurality of suspension members, a pair of tensionmembers connected between said Weight and said frame and means forvarying the tension of said tension members, according to the differencebetween a fixed frequency and the sum of the natural frequencies of saidtension members, and means for vibrating said tension members at theirnatural frequency, means for obtaining the sum of the naturalfrequencies -of said tension members and means for energizing saidtension varying means to keep the sum of said natural frequencies at aconstant value.

3. In a device of the character described, a frame, a weight, a pair oftension members connected between said weight and said frame, means forvibrating said tension members at their natural frequency and means forvarying the tension of said tension members in a manner to keep the sumof the natural frequencies at a constant value, means for obtaining thesum of the natural frequencies of said tension members and means forenergizing s-aid tension varying means to keep the sum of said naturalfrequencies at a constant value.

4. In a device of the character described, a frame, a

weight, a pair of tension members connected between said Weight and saidframe, means for vibrating said tension members at their natural.frequency and means for varying the tension of said tension members,comprising means for obtaining the sum of the natural frequencies ofsaid tension members, a standard frequency, means for obtaining thedifference between said standard frequency and the sum of saidfrequencies and means for energizing said tension varying meansaccording to said difference whereby said sum frequency is maintained ata constant value.

5. In a device of the character described, a frame including a pair ofend pieces and magnetostrictive legs between said end pieces, a weightsuspended in said frame by a plurality of suspension members, a pair oftension members connected between said weight and said end pieces andelectrical means for varying the tension of said tension members,according to the difference between a fixed frequency and the sum of thenatural frequencies of said tension members, said means comprising saidmagnetostrictive legs and means for applying a magnetic field to saidmagnetostrictive legs.

6. In a device of the character described, a frame including a pair ofend pieces and magnetostrictive legs between said end pieces, a weightsuspended in said frame by a plurality of suspension members, a pair oftension members connected between said weight and said frame andelectromechanical means for varying the tension of said tension members,according to the difference between a fixed frequency and the sum of thenatural frequencies of said tension members, said means comprising saidmagnetostri'ctive legs and means for applying a magnetic field to saidmagnetostrictive legs.

References Cited UNITED STATES PATENTS 1,948,104 2/ 1934 Firestone etal. 2,466,018 4/1949 Ferrill 310-26 X 2,725,492 11/1955 Allan 2641 XFOREIGN PATENTS 729, 894 12/1942 Germany.

RICHARD C. QUEISSER, Primary Examiner.

A. BLUM, SAMUEL BOYD, A. M. HORTON,

Examiners.

J. J. GILL, W. J. CURRAN, Assistant Examiners.

1. IN A DECIVE OF THE CHARACTER DESCRIBED, A FRAME, A WEIGHT SUSPENDEDIN SAID FRAME BY A PLURALITY OF SUSPENSION MEMBERS, A PAIR OF TENSIONMEMBERS CONNECTED BETWEEN SAID WEIGHT AND SAID FRAME AND MEANS FORVARYING THE TENSION OF SAID TENSION MEMBERS, ACCORDING TO THE DIFFERENCEBETWEEN A FIXED FREQUENCY AND THE SUM OF THE NATURAL FREQUENCIES OF SAIDTENSION MEMBERS, MEANS FOR OBTAINING THE SUM OF THE NATURAL FREQUENCIESOF SAID TENSION MEMBERS AND MEANS FOR ENERGIZING SAID TENSION VARYINGMEANS TO KEEP THE SUM OF SAID NATURAL FREQUENCIES AT A CONSTANT VALUE.